Electrical resistance heating is an effective and energy efficient method for heating electrically conductive articles, such as metal workpieces. In this process, an electrical current is flowed through the workpiece. By controlling the magnitude of the current, the amount of heating may be readily regulated.
In general, resistance heating methods provide for a reliable and uniform beating of workpieces; however, particular problems can be presented by workpieces which have an irregular shape or include openings, thinned portions, or other features therein. These irregularities can disrupt the current path and prevent uniform heating from occurring. For example, openings can result in a concentration of current proximate their perimeters, and thereby cause hotspots. Likewise, a thinned cross-sectional or transverse portion of an article can concentrate current and create hotspots. Establishing a uniform flow of current through an irregularly shaped workpiece is often difficult, since current tends to take the shortest path through the workpiece; and hence, particular regions may be “shaded” and under heated. This problem is particularly significant when relatively thin workpieces, such as structural components of motor vehicles, are being fabricated.
In response to the above stated problem, Arnosky in U.S. Pat. No. 3,737,618 disclosed a two-stage process for heating a slotted tube. In a first stage of the process, four sets of electrodes are attached to the end portions of the tube and energized in order to preheat the end regions. In a second stage, two of the sets of electrodes are removed from the tube so that the end regions and the slot-like area are heated simultaneously. Also, Gomez, in U.S. Pat. No. 6,897,407, discloses a two-stage process in which a first set of electrodes preheat a workpiece while a second set of shunting electrodes allows current to bypass small cross section regions of the workpiece. The first stage involves passing current through the workpiece with the smaller regions conductively bridged so that said regions were heated to a lesser extent. In the second stage, the shunting electrodes are removed and the entire workpiece is heated.
Both of the above prior art methods involve multiple stage processing on symmetrical workpieces. Hence, these systems cannot provide the control necessary for beating highly irregular pieces; and, the two-stage nature of the prior art processes complicates their use in high-speed automated processes. Thus, it will be appreciated that there is a need for a one-stage method and apparatus whereby electrical resistance heating may be utilized to uniformly heat workpieces which include irregular features and wherein a measured parameter of the workpiece aids in controlling the heating system. As will be explained in detail hereinbelow, the present invention meets this need.